Symmetrical, direct coupled laser drivers

ABSTRACT

Symmetrical, direct coupled laser drivers for high frequency applications. The laser drivers are in integrated circuit form and use a minimum of relatively small (low valued) external components for driving a laser diode coupled to the laser driver through transmission lines. An optional amplifier may be used to fix the voltage at an internal node at data frequency spectrum to improve circuit performance. Feedback to a bias input may also be used to fix the voltage at the internal node. Programmability and a burst mode capability may be included.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/440,539 filed Feb. 8, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of laser drivers, and moreparticularly to laser drivers for high frequency applications.

2. Prior Art

Laser drivers are well known in the prior art. However, currentoperating requirements at increased frequencies, lower voltages andhigher efficiencies exceed the performance of current designs.Representative prior art laser driver designs may be found in U.S. Pat.Nos. 7,181,100 and 7,457,336, US Published Application Nos. 2009/0268767and 2009/0201052. Some products currently on the market are described indata sheets MAX3656 and MAX3946 (Maxim Integrated Products, Inc.),ONET4201LD (Texas Instruments) and ADN2526 (Analog Devices).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is simplified circuit diagram of an embodiment of the presentinvention.

FIG. 2 is simplified circuit diagram of another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a circuit diagram for a preferred embodiment of thepresent invention may be seen. The circuitry in the rectangular outlinelabeled “External” is the only external circuitry needed, with the othercircuitry shown being within a single integrated circuit. The externalcircuitry includes the laser diode LD coupled to the integrated circuitterminals OUTA and OUTC through transmission lines TL1 and TL2. Aninductor L1 is connected between integrated circuit terminals VCC andOUTA, and a bypass capacitor is connected between VCC and the circuitground. An inductor L2 is connected between the OUTC and the VBIASterminals, and capacitor C2 is connected between the VBIAS terminal andthe circuit ground.

The outputs OUTA and OUTC coupled through the transmission lines to theanode and cathode of the laser diode LD, respectively, are connected tothe collectors of transistors T3 and T2, respectively. Transistor T3,biased by the bias voltage vcb, is a cascode transistor for transistorT1 of the differential transistor pair T1 and T2, which have theiremitters coupled together and to ground through resistor R3. Thecollector of transistor T3 is coupled to the anode connection of laserdiode LD through output terminal OUTA and transmission line TL1, and tothe VCC terminal of the integrated circuit through resistor R1 andexternal inductor L1. No cascode transistor is used for transistor T2 ofthe differential pair because of the lack of voltage headroom, theheadroom for cascode transistor T3 effectively being provided by thevoltage drop across laser diode LD itself. Note that the drive providedby transistors T1 and T2 is a symmetrical differential drive for thelaser diode LD.

The inputs to the bases of the differential pair T1 and T2 are thedifferential data inputs mod−′ and mod+′, respectively, which are theoutputs of amplifier A4. Amplifier A4 provides a plus and minusdifferential output having a fixed differential voltage, with voltagesset by the programmable I mod level block and with a state responsive tothe inputs mod+ and mod−. With this connection, the transistor that ison at any one time (T1 or T2) will conduct a current equal to its basevoltage minus its Vbe, all divided by the R3. Thus transistors T1 and T2act as current sources so that rather than being on and off, each act asa current source or is off, responsive to the digital data inputs mod+and mod−. User programmability of circuits and control loops in generalare well known in the art, as is the biasing of a transistor as acurrent source, and accordingly are not shown in detail herein. As analternative, the resistor R3 might be a programmable current source,though there may not be enough voltage headroom for such an embodiment,depending on the voltage VCC of the power source.

A second differential transistor pair T5 and T6 have their emittersconnected together and to the circuit ground through resistor R4. Thecollector of transistor T5 is connected to cascode transistor T4 havingits base and collector connected in common with the base and collectorof cascode transistor T3, respectively. Transistor T6 has its collectorconnected through capacitor C3 to the VCC terminal of the integratedcircuit, and to resistor R2. The other end of resistor R2 is connectedto the collector of transistor T2 and to the cathode of the laser diodeLD through integrated circuit terminal OUTC and transmission line TL2.The bases of transistors T5 and T6 are coupled as burst enable inputsbias−′ and bias+′, respectively, from amplifier A1 which controls thebase voltages of transistors T5 and T6 responsive to the signals bias+and bias−, and at voltage levels set by the Programmable I bias levelblock. Thus transistors also act as separately user programmable currentsources like those of transistors T1 and T2 for the digital data inputsmod+ and mod−. In FIG. 1, the connections between the integrated circuitand the terminals VCC, OUTA, OUTC and VBIAS are shown as inductors, asthe inductance of these connections at the high frequencies at which thepresent invention operates is meaningful and should be taken intoconsideration.

In one embodiment, additional circuitry is added to the input circuitry,namely to override the data inputs mod− high and mod+ low when bias− ishigh and bias+ is low, independent of any actual data inputs the circuitmay receive. This allows multiple laser drivers to share a communicationchannel in a time division multiplexing scheme where only onetransmitter is enabled at any times. Of course the override isimmediately released when bias+ goes high and bias− goes low (outputburst enabled). As an example, such an override can be implementedvarious ways using simple logic functions.

The impedance of the typical laser diode is on the order of 5 to 10 ohmsdifferentially, so there is a substantial impedance mismatch between thetransmission lines (25 ohm transmission lines or 50 ohm differentiallyin one embodiment) coupled to the laser diode LD. However, resistors R1and R2 are chosen to match the transmission lines, and provide thetermination for the signal reflected back from the laser diode LDthrough the transmission lines, resistor R1 being directly connected toVCC and R2 being AC coupled to VCC through capacitor C3 at thefrequencies of operation. Thus there is symmetry in the externalcircuitry as well as in the drive and termination of the 25 ohmtransmission lines.

When the output burst is not enabled (and mod− is held high and mod+ isheld low as previously described), there is no current through the laserdiode LD as there is no DC connection to its cathode. Thus there is nolight emission from the laser diode. The voltage at OUTA is essentiallyVCC, i.e., the voltage across inductor L1 is essentially zero because ofits low resistance. When the output burst is enabled and mod+ is highfor transmitting a “1”, current flows through the laser diode LD, with aprimary current path through transistor T2 and resistor R3 to thecircuit ground, transistor T2 acting as a current source controlled bythe output voltage levels of amplifier A4, which in turn provides outputvoltage levels controlled by the programmable I mod level block).

When the output burst is enabled and Mod− is high for transmission of a“0”, transistor T2 will be off. Now the component of the current throughthe laser diode LD and transistor T2 is off, so that the only remainingcomponent of current is that though resistor R2, transistor T6 andresistor R4. Note that in essence, transistors T2 and T1 steer thecurrent component of the current sources of transistors T2 and T1through the laser diode LD through and around the laser diode,respectively. This in turn means that the average current throughexternal inductor L1 is constant, independent of whether a “1” or a “0”is being transmitted.

It may be seen from the foregoing that the structure of the laser driverjust described and illustrated in FIG. 1 is a symmetrical, differentialdirect coupled laser driver structure with burst mode capabilities. Ofparticular interest is the fact that the data and bias loops are coupledtogether, the current for the transmission of a “1” being coupledthrough the bias loop.

Circuit performance may be optionally improved by actively fixing thevoltage of the node VBIAS, at least with respect to high frequencysignals (i.e., signals in the data frequency spectrum, as opposed to lowfrequencies which are frequencies well below the data frequencyspectrum). For this purpose, the optional circuit in the box at the leftof FIG. 1 can be added. This circuit is, in essence, an amplifier A(s)with a complex transfer function and with a feedback resistor R7 asshown in the oval outline in FIG. 1. It has an output impedance for theoutput VBIAS that is very low at the high frequencies, i.e., looks likea voltage source, but which is very high for low frequencies, i.e.,looks like a current source. At high frequencies, it can be consideredto make capacitor C3 look very large, thereby improving the terminationof the cathode and thus the circuit balance. At low frequencies, thecircuit will seek an output VBIAS equal to the average voltage on thenode to which it is connected.

Now referring to the circuit to the left of FIG. 1 within the outline,transistor T7 is coupled through resistor R5 as an emitter followerbiased by current source I1. The circuit to the left of transistor T7 isan amplifier responsive at high frequencies to changes in the voltage onthe emitter of transistor T7, the voltage VBIAS, by feedback throughresistor R7, to adjust the voltage on the base of transistor T7 tocancel or greatly reduce such changes.

In particular, a change in the voltage VBIAS changes the current throughtransistor T8, changing the voltage drop across resistor R6, couplingthat change through the base emitter voltage of transistor T9 biased bycurrent source I3 to the base of transistor T11. This changes thecurrent through transistor T11 which causes the voltage drop throughresistor R8 to change, feeding this change back to the base oftransistor T7 to resist the change in VBIAS that initiated thedisturbance.

The bases of transistors T7 and T8 are coupled to VCC through resistorsR8 and R9, respectively, and to the collectors of differentialtransistor pair T11 and T10, respectively. The base of transistor T10 isconnected to a reference voltage, with the common emitter connection oftransistors T10 and T11 being biased by the current through transistorT12. The voltage on capacitor C6 connected to the gate of transistor T12integrates the output of the voltage controlled current source I4(transconductance Gm), which is proportional to the difference involtages on the collectors of transistors T7 and T8. If at a lowfrequency (relative to the data frequencies), VBIAS changes, that willcause a current through resistor R5, which in turn will cause a changein voltage on capacitor C4. That causes a voltage difference acrossvoltage controlled current source I4, unbalancing the voltage across thevoltage controlled current source I4 to charge or discharge capacitor C6until the circuit settles at the new value of VBIAS.

Capacitor C4 provides a capacitive load on the collector of transistorT7 at data frequencies to limit the fluctuations of the voltage on thecollector of transistor T7 to avoid saturation of the transistor.Capacitor C5, on the other hand, determines the circuit response tofrequencies in a mid-frequency range.

As an alternative to the circuit at the left of FIG. 1, the disturbancesin VBIAS at data frequencies may be substantially eliminated using thecircuit of FIG. 2. This circuit, like that of FIG. 1, senses anyattempted change in VBIAS at data frequencies, but in addition todirectly adjusting VBIAS, provides feedback to transistors T5 and T6 sothat transistor T6 will also provide (or absorb) the current componentthat is attempting to vary VBIAS at the data frequencies. The end resultis the same as that achieved by the circuit at the left of FIG. 1,though the feedback is both directly to VBIAS itself as in the circuitof FIG. 1, and indirectly to VBIAS through transistor T6.

Referring again to FIG. 2, resistors R4, R5 and R7, transistors T5, T6and T7, current source I1 and signals bias+ and bias− are the same as inFIG. 1, the rest of the right side of FIG. 1 being omitted in FIG. 2 forclarity, though could be identical to that of FIG. 1. In operation,resistor R7 senses any change in the VBIAS voltage, which change isamplified by amplifier A′(s) and fed back to the base of transistor T7.Amplifier A′(s) can have a transfer function f(s) that produces the sameVBIAS voltage characteristics (acts like a voltage source at datafrequencies and as a current source at low frequencies) as the circuitin the box at the left of FIG. 1. Actually taking out amplifiers A2 andA3, the circuit shown in FIG. 2 may be considered a simplifiedrepresentation of the corresponding part of the circuit of FIG. 1.

Amplifier A1 is shown in FIG. 1, though with only one programmable inputfor bias control. In FIG. 2, additional inputs are provided for controlthrough amplifiers A2 and/or A3. Normally both path 2 and path 3 wouldnot be used, and without path 2 and path 3, but only path 1, one has asimplified version of the respective part of the circuit of FIG. 1.Again, paths 1, 2 and 3 may be eliminated, though performance isimproved and the size of the external components, especially inductor L2and capacitor C2 may be reduced by using at least feedback path 1, aspreviously described. Also if path 2 is used (i.e., paths 1 and 2 areused), then capacitor C4 may be eliminated, or substantially reduced incapacitance.

The present invention has been disclosed and described with respect tonpn transistors, though it should be noted that it can be implemented inother technologies, such as by way of example, using NMOS transistors.In any case, each transistor, regardless of type, may be characterizedas having first, second and third terminals wherein the voltage betweenthe first (emitter or source) and second terminals (base or gate)controls the conduction of current between the third (collector ordrain) and first (emitter or source) terminals. Also transistors T3 andT4 are cascode transistors, and may be eliminated if desired, as long astransistors T1 and T5 can handle the higher voltage that they will besubjected to. In that regard, the topology of the invention may beinverted and implemented with transistors of the opposite conductivitytype.

There have been disclosed herein symmetrical, differential directcoupled laser driver structures with burst mode capabilities thatoperate on low supply voltages with high efficiency. While certainpreferred embodiments of the present invention have been disclosed anddescribed herein for purposes of illustration and not for purposes oflimitation, it will be understood by those skilled in the art thatvarious changes in form and detail may be made therein without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A laser diode driver comprising: a power supplyconnection, a laser diode anode connection, a laser diode cathodeconnection, a first bias connection and a circuit ground connection; afirst inductor coupled between the power supply connection and the laserdiode anode connection; a second inductor coupled between the laserdiode cathode connection and the first bias connection; and a firstcapacitor coupled between the first bias connection and the circuitground connection; a first resistor coupled between the power supplyconnection and the laser diode anode connection; a second resistorcoupled between the laser diode cathode connection and the first biasconnection; a current source coupled between the first bias connectionand the circuit ground connection; first and second transistors, eachhaving first, second and third terminals wherein the voltage between thefirst and second terminals controls the conduction between the third andfirst terminals, the first and second transistors having their firstterminals coupled together and to the circuit ground connection, thethird terminal of the first transistor being coupled to the laser diodeanode connection, the second terminals of the first and secondtransistors being coupled to a first differential voltage sourceresponsive to differential data inputs to controllably turn on either ofthe first and second transistors as a current source of a firstdifferential current source and to turn off the other of the first andsecond transistors as defined by the differential data inputs; theextent to which the differential data inputs turn on either of the firstand second transistors as a current source of the first differentialcurrent source and to turn off the other of the first and secondtransistors as defined by the differential data inputs is programmable;the second transistor having its third terminal coupled to the laserdiode cathode connection.
 2. The laser diode driver of claim 1 furthercomprising a second capacitor coupled between the power supplyconnection and the circuit ground connection.
 3. A laser diode drivercomprising: a power supply connection, a laser diode anode connection, alaser diode cathode connection, a first bias connection and a circuitground connection; a first resistor coupled between the power supplyconnection and the laser diode anode connection; a second resistorcoupled between the laser diode cathode connection and the first biasconnection; a current source coupled between the first bias connectionand the circuit ground connection; first, second and third transistors,each having first, second and third terminals wherein the voltagebetween the first and second terminals controls the conduction betweenthe third and first terminals, the first and second transistors havingtheir first terminals coupled together and to the circuit groundconnection, the third terminal of the first transistor being coupled tothe laser diode anode connection, the second terminals of the first andsecond transistors being coupled to a first differential voltage sourceresponsive to differential data inputs to controllably turn on either ofthe first and second transistors as a current source of a firstdifferential current source and to turn off the other of the first andsecond transistors as defined by the differential data inputs; thesecond transistor having its third terminal coupled to the laser diodecathode connection; the third transistor having its first terminalcoupled to the third terminal of the first transistor, its secondterminal coupled to a second bias connection and its third terminalcoupled to the laser diode anode connection, whereby the third terminalof the first transistor is coupled to the laser diode anode connectionthrough the third transistor.
 4. The laser diode driver of claim 3wherein the current source coupled between the first bias connection andthe circuit ground connection comprises: fourth, fifth and sixthtransistors, each having first, second and third terminals wherein thevoltage between the first and second terminals controls the conductionbetween the third and first terminals, the fourth transistor having itsthird terminal coupled to the laser diode anode connection, its secondterminal coupled to the second bias connection and its first terminalcoupled to the third terminal of the fifth transistor, the thirdterminal of the sixth transistor being coupled to the first biasconnection, the first terminals of the fifth and sixth transistors beingcoupled to the circuit ground connection, and the second terminals ofthe fifth and sixth transistors being coupled to a second differentialvoltage source responsive to differential burst enable inputs tocontrollably turn on either of the fifth and sixth transistors as thecurrent source of a second differential current source and to turn offthe other of the fifth and sixth transistors as defined by thedifferential burst enable inputs, the sixth transistor when on, actingas the current source coupled between the first bias connection and thecircuit ground connection.
 5. The laser diode driver of claim 4 furthercomprised of an amplifier having an output coupled to the first biasconnection, the amplifier having a complex transfer function providing alow output impedance at high frequencies and a high output impedance atlow frequencies.
 6. The laser diode driver of claim 5 wherein theamplifier includes a seventh transistor having first, second and thirdterminals wherein the voltage between the first and second terminalscontrols the conduction between the third and first terminals, andwherein the seventh transistor is coupled as a first terminal followeroutput of the amplifier.
 7. The laser diode driver of claim 6 whereinthe second differential current source is coupled to be responsive tothe voltage on the first terminal of the seventh transistor to resistvoltage changes in a frequency spectrum of the differential data inputson the output of the amplifier.
 8. The laser diode driver of claim 6further comprising: a first inductor coupled between the power supplyconnection and the laser diode anode connection; a second inductorcoupled between the laser diode cathode connection and the first biasconnection; and a first capacitor coupled between the first biasconnection and the circuit ground connection.
 9. A laser diode drivercomprising: a power supply connection, a laser diode anode connection, alaser diode cathode connection, a first bias connection and a circuitground connection; a first resistor coupled between the power supplyconnection and the laser diode anode connection; a second resistorcoupled between the laser diode cathode connection and the first biasconnection; a current source coupled between the first bias connectionand the circuit ground connection; first through fourth transistors,each having first, second and third terminals wherein the voltagebetween the first and second terminals controls the conduction betweenthe third and first terminals, the first and second transistors havingtheir first terminals coupled together and to the circuit groundconnection, the third terminal of the first transistor being coupled tothe laser diode anode connection, the second terminals of the first andsecond transistors being coupled to a first differential voltage sourceresponsive to differential data inputs to controllably turn on either ofthe first and second transistors as a current source of a firstdifferential current source and to turn off the other of the first andsecond transistors as defined by the differential data inputs; thesecond transistor having its third terminal coupled to the laser diodecathode connection; wherein the current source coupled between the firstbias connection and the circuit ground connection comprises: the firstterminals of the third and fourth transistors being coupled to thecircuit ground connection, and the second terminals of the third andfourth transistors being coupled to a second differential voltage sourceresponsive to differential burst enable inputs to controllably turn oneither of the third and fourth transistors as the current source of asecond differential current source and to turn off the other of thethird and fourth transistors as defined by the differential burst enableinputs, the fourth transistor when on, acting as the current sourcecoupled between the first bias connection and the circuit groundconnection.
 10. The laser diode driver of claim 9 wherein the first andsecond differential voltage sources are programmable.
 11. The laserdiode driver of claim 10 further comprising a capacitor coupled betweenthe first bias connection and the power supply connection.
 12. The laserdiode driver of claim 9 further comprising a capacitor coupled betweenthe first bias connection and the power supply connection.
 13. The laserdiode driver of claim 9 further comprising: a first inductor coupledbetween the power supply connection and the laser diode anodeconnection; a second inductor coupled between the laser diode cathodeconnection and the first bias connection; and a first capacitor coupledbetween the first bias connection and the circuit ground connection. 14.A laser diode driver comprising: a power supply connection, a laserdiode anode connection, a laser diode cathode connection, a first biasconnection and a circuit ground connection; a first resistor coupledbetween the power supply connection and the laser diode anodeconnection; a second resistor coupled between the laser diode cathodeconnection and the first bias connection; a current source coupledbetween the first bias connection and the circuit ground connection;first, second and third transistors, each having first, second and thirdterminals wherein the voltage between the first and second terminalscontrols the conduction between the third and first terminals, the firstand second transistors having their second terminals coupled to beresponsive to differential data inputs to controllably turn on either ofthe first and second transistors as a current source of a firstdifferential current source and to turn off the other of the first andsecond transistors as defined by the differential data inputs; the firsttransistor having its third terminal to the anode connection and thesecond transistor having its third terminal coupled to the laser diodecathode connection; the third transistor having its first terminalcoupled to the third terminal of the first transistor, its secondterminal coupled to a second bias voltage and its third terminal coupledto the laser diode anode connection, whereby the third connection of thefirst transistor is coupled to the anode connection through the thirdtransistor.
 15. The laser diode driver of claim 14 further comprising: afirst inductor coupled between the power supply connection and the laserdiode anode connection; a second inductor coupled between the laserdiode cathode connection and the first bias connection; and a firstcapacitor coupled between the first bias connection and the circuitground connection.
 16. The laser diode driver of claim 14 wherein thefirst differential current source is programmable.
 17. A laser diodedriver comprising: a power supply connection, a laser diode anodeconnection, a laser diode cathode connection, a first bias connectionand a circuit ground connection; a first resistor coupled between thepower supply connection and the laser diode anode connection; a secondresistor coupled between the laser diode cathode connection and thefirst bias connection; a current source coupled between the first biasconnection and the circuit ground connection; first through fourthtransistors, each having first, second and third terminals wherein thevoltage between the first and second terminals controls the conductionbetween the third and first terminals, the first and second transistorshaving their second terminals coupled to be responsive to differentialdata inputs to controllably turn on either of the first and secondtransistors as a current source of a first differential current sourceand to turn off the other of the first and second transistors as definedby the differential data inputs; the first transistor having its thirdterminal to the anode connection and the second transistor having itsthird terminal coupled to the laser diode cathode connection; whereinthe current source coupled between the first bias connection and thecircuit ground connection comprises: the first terminals of the thirdand fourth transistors being coupled to the circuit ground connection,and the second terminals of the third and fourth transistors beingcoupled to differential burst enable inputs to controllably turn oneither of the third and fourth transistors as the current source of asecond differential current source and to turn off the other of thethird and fourth transistors as defined by the differential burst enableinputs, the fourth transistor when on, acting as the current sourcecoupled between the first bias connection and the circuit groundconnection.
 18. The laser diode driver of claim 17 wherein the seconddifferential current source is programmable.
 19. The laser diode driverof claim 17 further comprised of an amplifier having an output coupledto the first bias connection, the amplifier having a complex transferfunction providing a low output impedance at high frequencies and a highoutput impedance at low frequencies.
 20. The laser diode driver of claim19 further comprised of a fifth transistor having first, second andthird terminals wherein the voltage between the first and secondterminals controls the conduction between the third and first terminals,the fifth transistor being coupled as a first terminal follower outputof the amplifier.
 21. The laser diode driver of claim 20 wherein thesecond differential current source is coupled to be responsive to thevoltage on the first terminal of the fifth transistor to resist voltagechanges at data spectrum frequencies on the output of the amplifier. 22.The laser diode driver of claim 20 wherein the second differentialcurrent source is coupled to be responsive to the voltage on the firstterminal of the fifth transistor to resist voltage changes at data rateson the output of the amplifier.
 23. The laser diode driver of claim 20further comprising: a first inductor coupled between the power supplyconnection and the laser diode anode connection; a first capacitorcoupled between the power supply connection and the circuit groundconnection; a second inductor coupled between the laser diode cathodeconnection and the first bias connection; and a second capacitor coupledbetween the first bias connection and the circuit ground connection.